Stephen J. Gallagher

Origin and Timing of the Miocene-Pliocene Unconformity in Southeast Australia

An unconformity is present close to the Miocene-Pliocene boundary in the onshore and nearshore portions of the Otway, Port Phillip-Torquay, and Gippsland basins of southeast Australia. The unconformity is angular (generally < 1-5° angularity), with the underlying Miocene units having been deformed (gentle folding and reverse faulting) and eroded prior to deposition of the Pliocene succession. The unconformity also marks a change from Oligocene-Miocene deposition of cool-water carbonate sediments and brown coal-bearing successions to the accumulation of more siliciclastic-rich sediments in Pliocene time. The Miocene-Pliocene boundary therefore represents an interval of significant regional uplift in the southeast Australian basins. The amount of section removed is greatest around the Otway and Strzelecki ranges in Victoria, where up to a kilometer of section may have been removed. In most onshore sections of the Victorian basins hundreds of meters of section have been eroded. In distal offshore locations the boundary becomes conformable. The timing of uplift and erosion is best constrained in the Otway and Port Phillip basins, where late Miocene (N16 ∼ 10 Ma) sediments underlie the unconformity and earliest Pliocene (∼ 5 Ma) sediments overlie it. This timing coincides with a change in the dynamics of the Australian plate, beginning at around 12 Ma. Southeast Australia is currently under a NW-SE compressional regime, and this has probably persisted since the late Miocene. In the basins (as opposed to the basement-dominated highland areas), the late Miocene uplift event is more significant than later Pliocene-Recent uplift.

Cenozoic stratigraphic succession in southeastern Australia

Strata of Cenozoic age occur around the southern margin of Australia as thin and discontinuous outcrops, interpolated and fleshed out by economic exploration onshore and offshore. The neritic strata fall into four sequences or allostratigraphic packages of (I) Paleocene — Early Eocene, (II) Middle Eocene — Early Oligocene, (III) Late Oligocene — Middle Miocene, and (IV) Late Miocene — Holocene age: a four‐part pattern that can be seen also in the flanking pelagic and terrestrial realms including regolith deep weathering. Problems of correlation and age determination (predominantly biostratigraphic) have included biogeographical constraints (endemism in neritic molluscs and terrestrial palynomorphs, mid‐latitude assemblages in calcareous plankton), and slow progress in magnetostratigraphy and chemostratigraphy. Sequence I largely repeats the Cretaceous siliciclastic‐coal, marginal‐marine facies (carbonate‐poor, with marine and non‐marine palynomorphs and agglutinated foraminifers) punctuated by marine ingressions with microfaunas and sparse macrofaunas. Sequence II contains the first carbonates in the region since the Palaeozoic and the most extensive coals of the Cenozoic anywhere. Sequence III contains the most extensive neritic carbonates and the last major coals. Sequence IV is more strongly siliciclastic than the two preceding. Each of these four second‐order entities (107 years duration) comprises third‐order packages each with an unconformity and marine transgression. These packages hold true right along the southern Australian margin in the sense that the hiatuses and transgression do not display significant diachroneity at the relevant time‐scales (105–106 years). Recognised, delimited and correlated independently of the putatively global Exxon sequences, they are remarkably consistent with the latter, thereby providing a significant regional test. There are two widespread emphases on southern Australian geohistory and biohistory: (i) to regard the regional story as part of the global story of accreting continents, an expiring Tethys, and an episodically cooling planet; and (ii) a somewhat contrary emphasis, with the region being a special case of rapid longitudinal motion towards the equator. Both emphases are plausible with the former being the more heuristic. The stratigraphic record is strongly punctuated, the four sequences being separated by both tectonic and climatic events. Thus: the sequence I/II gap involved extensive plate‐tectonic reorganisation and a new spreading regime from ca 43 Ma, coevally with early growth of Antarctic ice; in the II/III gap, deformation in marginal basins is coeval with a global low in cooling, large ice sheet and falling sea‐level to ca 30 Ma; and the III/IV gap is marked by widespread cessation or contraction of stratal accumulation and withdrawal of thermophilic taxa coevally with the major expansion at ca 14 Ma of the Antarctic ice sheet, onset of intense canyon cutting, and plate‐wide basin inversion.

The Pliocene climatic and environmental evolution of southeastern Australia: evidence from the marine and terrestrial realm

Abstract During the Pliocene the global climate fluctuated markedly with the expansion and contraction of the Northern and Southern Hemisphere ice sheets. The signals of this change are well preserved in the thick (up to 1 km) Seaspray Group cool-water carbonate sediments in the Gippsland region and associated thin terrestrial deposits in southeastern Australia. This study uses seismic, facies, foraminiferal proxy data and palaeobotanical data to chart the Pliocene climate and environmental change in the marine and terrestrial realms of southeastern Australia. Complex submarine canyoning occurred at the shelf/upper bathyal transition during the Pliocene in Gippsland. Low-energy pelagic marl (wackestone/packstone) characterise canyon and inter-canyon environments in the earliest Pliocene, depositing plankton oozes with interbedded calciturbidites. From upper Early Pliocene to Late Pliocene time high-energy limestone (grainstone) facies infilled these submarine canyons associated with progradation of the succession from outer to middle shelf palaeoenvironments. Plankton proxy data suggest cool conditions in the basal part of Early Pliocene. Relatively stable warmer marine conditions prevailed throughout most of the Early Pliocene, corresponding to a period of globally low δ18O values in the oceans associated with minor Antarctic ice sheet expansion. From middle to Late Pliocene time marked fluctuations in the abundance of cool and warmer water plankton taxa occurred, corresponding to a time of global marine δ18O fluctuations and generally heavier δ18O values associated with the expansion of the Antarctic ice and Northern Hemisphere ice sheets. Upwelling is interpreted to have occurred throughout much of the warmer Early Pliocene, caused by a more northerly (compared to today) positioned and weaker Subtropical Front. Upwelling was prevalent in the outer shelf to upper slope facies at the ‘palaeo’ Bass Canyon and the Subtropical Front migrated northwards to Gippsland during Late Pliocene glacial periods. Terrestrial palaeobotanical records indicate a shift from widespread Araucarian forests and rainforest, including ‘tropical’ taxa now extinct in the region, to a landscape by the end of the Late Pliocene similar to that of the present day with a mosaic of Eucalyptus–Acacia–Casuarinaceae sclerophyllous forests and open vegetation, with local areas of Nothofagus-dominated cool temperate rainforests. Palaeobotanical proxy data indicate that regional climate oscillated between warm–wet and cool–dry phases, with an overall cooling–drying trend through the Pliocene. Earliest Pliocene climates in southeast Australia were warm–wet with a summer rainfall peak (mean annual temperature, MAT, 2–4°C higher than present, mean annual precipitation, MAP, 50–70% higher than present), whereas terminal Late Pliocene climates were drier–cooler with a winter rainfall peak (MAT 0–2°C higher than present, MAP 0–30% higher than present).

NEOGENE TECTONICS IN SE AUSTRALIA: IMPLICATIONS FOR PETROLEUM SYSTEMS

The influence of Neogene tectonics in the SE Australian basins has generally been underestimated in the petroleum exploration literature. However, onshore stratigraphic and offshore seismic data indicates that significant deformation and exhumation (up to one km or more) has occurred during the late Tertiary-Quaternary. This tectonism coincides with a change in the dynamics of the Australian plate, beginning at around 12 Ma, resulting in a WNW–ESE compressional regime which has continued to the present day. Significant late Miocene tectonism is indicated by a regional angular unconformity at around the Mio-Pliocene boundary in the onshore and nearshore successions of the SE Australian basins. Evidence of on going Pliocene- Quaternary tectonism is widespread in all of the SE Australian basins. Late Tertiary tectonism has produced structures in the offshore SE Australian basins which have been favourable targets for petroleum accumulation (e.g. Nerita structure, Torquay Sub-basin; Cormorant structure, Bass Basin). In the offshore Gippsland Basin, most of the oil- and gas-bearing structures have grown during Oligocene-Recent time. Some Gippsland Basin structures were largely produced prior to the mid- Miocene, while others have a younger structural history. In areas of intense late Tertiary exhumation and uplift (e.g. proximal to the Otway and Strzelecki Ranges), burial/maturation models of petroleum generation may be significantly affected by Neogene uplift. Many structures produced by late Miocene-Pliocene deformation are dry. These relatively young structures may post-date the major maturation episodes, with the post-structure history of the basins dominated by exhumation and cooling.

No mountains to snow on: major post-Eocene uplift of the East Victoria Highlands; evidence from Cenozoic deposits

Since the idea of the Pliocene Kosciusko Uplift in the Southeastern Highlands of Australia was first introduced, there has been considerable debate about the validity of this Cenozoic uplift event. Until the mid 1990s, most researchers argued that most highland relief was present by the Cretaceous. Since the late 1990s, there has been a paradigm shift that extensive young Cenozoic uplift created much of the high relief. In this paper, we synthesise Cenozoic stratigraphic and structural data from the East Victoria Highlands to assess the timing and origin of uplift. New high-resolution radar topography data indicate extensive east-northeast- and northeast-trending vertical and horizontal fault block displacement of the Cenozoic volcanic and sedimentary paleovalley infill. We suggest that regional uplift and exhumation of the East Victoria Highlands took place along these faults, initiated during the Late Eocene to Early Oligocene, and movements continue to the present day. By a combination of block faulting and epeirogenic uplift the divide migrated 40 km north reaching the present position by Pliocene time. Paleocurrent, lateral stream and magnetic basaltic valley flow directions indicate northward paleoflow directions for many of the Eocene – Oligocene valleys even those south of the present divide. Paleovalleys close to the Gippsland Basin show southward flow directions. The uplift that began in the Eocene causing valley cut and infill, eroded an Early Cenozoic paleoplain surface. Remnants of Late Eocene to Oligocene ligneous sediments are preserved as sub-basaltic, lowland valley, fluvio-lacustrinal sediment on this surface. Three large low-gradient paleodrainage systems that begin south of the present divide flowed north over 100 km to the Murray Basin where they are overlain by younger sediments. In contrast, paleodrainage systems flowing south from the present coastal escarpment to Gippsland, were shorter and steeper. The similarities of palynofacies of this infill to the adjacent basins suggest the valleys were low-relief/low-altitude paleodrainage systems that extended over the East Highlands. Based on our palynology results from Mt Hotham (present-day height 1800 m), previous macrofossil estimates of 800 m maximum Eocene relief could be overestimates of the paleo-height. Due to Cenozoic uplift, the strata (where present) are preserved as hilltop deposits and flows tilted away from the present divide. The Late Eocene to Pliocene uplift is probably primarily responsible for the topographic relief of the present East Victoria Highlands, is of the order of several hundreds of metres to a kilometre, and commenced in the Late Eocene at a divide closer to the Gippsland Basin than at present. This uplift continues to the present day as shown by the active seismicity in the area.

Late Dinantian (Lower Carboniferous) platform carbonate stratigraphy of the Buttevant area North Co. Cork, Ireland

A thick sequence of late Dinantian (Asbian–Brigantian) carbonates crop out in the Buttevant area, North Co. Cork, Ireland. A mud-mound unit of early Asbian age (the Hazelwood Formation) is the oldest unit described in this work. This formation is partly laterally equivalent to, and is overlain by, over 500 m of bedded platform carbonates which belong to the Ballyclogh and Liscarroll Limestone Formations. Four new lithostratigraphic units are described within the platform carbonates: (i) the early Asbian Cecilstown Member and (ii) the late Asbian Dromdowney Member in the Ballyclogh Limestone Formation; (iii) the Brigantian Templemary Member and (iv) the Coolbane Member in the Liscarroll Limestone Formation. The Cecilstown Member consists of cherty packstones and wackestones that are inferred to have been deposited below fair-weather wavebase. This unit overlies and is laterally equivalent to the mud-mound build-up facies of the Hazelwood Formation. The Dromdowney Member is typified by cyclic-bedded kamaenid-rich limestones possessing shell bands, capped by palaeokarst surfaces, with alveolar textures below and shales above these surfaces. The carbonates of this unit were deposited at or just below fair-weather wavebase, the top of each cycle culminated in subaerial emergence. The Templemary Member consists of cyclic alternations of subtidal crinoidal limestones capped by subtidal lagoonal crinoid-poor, peloidal limestones possessing coral thickets. Intraclastic cherty packstones and wackestones characterize the Coolbane Member, which is inferred to have been deposited below fair-weather wavebase but above storm wavebase. The early Asbian Cecilstown Member has a relatively sparse micro- and macrofauna, typified by scattered Siphonodendron thickets, archaediscids at angulatus stage and common Vissariotaxis. Conversely, macro- and microfauna is abundant in the late Asbian Dromdowney Member. Typical late Asbian macrofossils include the coral Dibunophyllum bipartitum and the brachiopod Davidsonina septosa. The base of the late Asbian (Cf6γ Subzone) is recognized by the first appearance of the foraminifers Cribrostomum lecompteii, Koskinobigenerina and the alga Ungdarella. The Cf6γ Subzone can be subdivided into two biostratigraphic divisions, Cf6γ1 and Cf6γ2, that can be correlated throughout Ireland. Relatively common gigantoproductid brachiopods and the coral Lonsdaleia duplicata occur in the Brigantian units. The base of the Brigantian stage (Cf6δ Subzone) is marked by an increase in the abundance of stellate archaediscids, the presence of Saccamminopsis-rich horizons, Loeblichia paraammonoides, Howchinia bradyana and the rarity of Koninckopora species. Changes in facies at the Cecilstown/Dromdowney Member and the Ballyclogh/Liscarroll Formation boundaries coincide closely with the changes in fossil assemblages that correspond to the early/late Asbian and the Asbian/Brigantian boundaries. These facies changes are believed to reflect major changes in relative sea-level on the Irish platforms. The sea-level variations that are inferred to have caused the facies changes at lithostratigraphic boundaries also brought in the new taxa that define biostratigraphic boundaries. Moreover, many of the Dinantian stage boundaries that are defined biostratigraphically in Great Britain, Belgium and the Russian Platform also coincide with major facies boundaries caused by regressive and transgressive episodes. The integration of detailed biostratigraphic analyses with facies studies will lead to better stratigraphic correlations of Dinantian rocks in northwest Europe.

The Pliocene to recent history of the Kuroshio and Tsushima Currents: a multi-proxy approach

The Kuroshio Current is a major western boundary current controlled by the North Pacific Gyre. It brings warm subtropical waters from the Indo-Pacific Warm Pool to Japan exerting a major control on Asian climate. The Tsushima Current is a Kuroshio offshoot transporting warm water into the Japan Sea. Various proxies are used to determine the paleohistory of these currents. Sedimentological proxies such as reefs, bedforms, sediment source and sorting reveal paleocurrent strength and latitude. Proxies such as coral and mollusc assemblages reveal past shelfal current activity. Microfossil assemblages and organic/inorganic geochemical analyses determine paleo- sea surface temperature and salinity histories. Transportation of tropical palynomorphs and migrations of Indo-Pacific species to Japanese waters also reveal paleocurrent activity. The stratigraphic distribution of these proxies suggests the Kuroshio Current reached its present latitude (35 °N) by ~3 Ma when temperatures were 1 to 2 °C lower than present. At this time a weak Tsushima Current broke through Tsushima Strait entering the Japan Sea. Similar oceanic conditions persisted until ~2 Ma when crustal stretching deepened the Tsushima Strait allowing inflow during every interglacial. The onset of stronger interglacial/glacial cycles ~1 Ma was associated with increased North Pacific Gyre and Kuroshio Current intensity. This triggered Ryukyu Reef expansion when reefs reached their present latitude (~31 °N), thereafter the reef front advanced (~31 °N) and retreated (~25 °N) with each cycle. Foraminiferal proxy data suggests eastward deflection of the Kuroshio Current from its present path at 24 °N into the Pacific Ocean due to East Taiwan Channel restriction during the Last Glacial Maximum. Subsequently Kuroshio flow resumed its present trajectory during the Holocene. Ocean modeling and geochemical proxies show that the Kuroshio Current path may have been similar during glacials and interglacials, however the glacial mode of this current remains controversial. Paleohistorical studies form important analogues for current behavior with future climate change, however, there are insufficient studies at present in the region that may be used for this purpose. Modeling of the response of the Kuroshio Current to future global warming reveals that current velocity may increase by up to 0.3 m/sec associated with a northward migration of the Kuroshio Extension.

Plio-Pleistocene tectonics and eustasy in the Gippsland Basin, southeast Australia: evidence from magnetic imagery and marine geological data

The Pliocene and Pleistocene sediments of the Gippsland shelf are dominated by mixed carbonates and siliciclastics. From a detailed stratigraphic study that combines conventional marine geology techniques with magnetic imagery, the Late Neogene tectonic and eustatic history can be interpreted and correlated to the onshore section. Stratigraphic analyses of eight oil and gasfield foundation bores drilled to 150 m below the seabed revealed three principal facies types: (i) Facies A is fine‐grained limestone and limey marl deeper than 50 m below the seabed, of Late Pliocene age (nannofossil zones CN11–12); (ii) Facies B is a fine‐coarse pebble quartz‐carbonate sand that occurs 10–50 m below the seabed in the inner shelf, grading down into Facies A in wells in the outer shelf, and is of Early‐Middle Pleistocene age (nannofossil subzones CN13a-14b: ca 1.95–0.26 Ma); and (iii) discontinuous horizons of Facies C composed of carbonate‐poor carbonaceous and micaceous fine quartz sand occurring 10–50 m below the seabed. The sparse benthic foraminifers in Facies C are inner shelf or Gippsland (euryhaline) Lakes forms. Holocene sands dominate the upper 1.5–2.5 m of the Gippsland shelf and disconformably overlie cemented limestones with aragonite dissolution, indicating previous exposure to meteoric water. Nannofossil dating of the limestones indicates ages within subzone CN14b (dated between ca 0.26 and 0.47 Ma). Airborne magnetic imaging across the Gippsland shelf and onshore provides details of buried magnetic palaeoriver channels and barrier systems. The river systems trend south‐southeast from the Snowy, Tambo, Mitchell, Avon, Macalister and Latrobe Rivers across the shelf. Sparker seismic surveys show the magnetic palaeochannels as seismic ‘smudges’ 20–40 m below the seabed. They appear to correspond to Facies C lenses (i.e. are Early to Middle Pleistocene features). Magnetic palaeobarrier systems trending south‐southwest in the inner shelf and onshore beneath the Gippsland Lakes are orientated 15° different to the modern Ninety Mile Beach barrier trend. Offshore, they correlate stratigraphically to progradation packages of Facies B. Analysis of bore data in the adjacent onshore Gippsland Lakes suggests that a Pliocene barrier sequence 100–120 m below surface is overlain by fluvial sand‐gravel and lacustrine mud facies. The ferruginous sandstone beds resemble offshore Facies C, and are located where magnetic palaeoriver channel systems occur, implying Early to Middle Pleistocene ages. Presence of the estuarine bivalve Anadara trapezia in the upper lacustrine mud facies suggests that the Gippsland Lakes/Ninety Mile Beach‐type barriers developed over the past 0.2 million years. Further inland, magnetic river channels that cut across present‐day uplifted structures, such as the Baragwanath Anticline, suggest that onshore Gippsland uplift continued into the Middle Pleistocene.

The stratigraphy and cyclicity of the late Dinantian platform carbonates in parts of southern and western Ireland

Abstract The late Dinantian platform carbonate successions of the Burren, Buttevant and Callan areas in Ireland have been correlated using detailed litho- and biostratigraphy. Several subdivisions or lithofacies associations (LA) of the Asbian to Brigantian part of the succession have been recognized in each area (lithofacies associations 1–5). Holkerian(?) to early Asbian ramp carbonates of LA 1 underlie the late Asbian successions in both the Burren and Buttevant areas. The influx of the bilaminar palaeotextulariid Cribrostomum lecomptei coincides with the onset of late Asbian shallow-marine cyclic platform sedimentation (LA 2) in all areas. The sediments of LA 2 have abundant Kamaenella and foraminifera, and are characterized by palaeokarstic surfaces and shales that cap a minimum of nine shallowing-upward minor cycles in the Burren and Buttevant areas. Two new biostratigraphic subdivisions (Cf6γ1 and Cf6γ2) of the late Asbian/Cf6γ subzone are described for the first time in this part of the succession. Brigantian sedimentation (LA 3) is typified by amalgamated beds of crinoidal limestone and abundant Asteroarchaediscidae. Thin peloidal limestones with coral thickets cap shallowing-upward cycles within this succession. These cycles, which are thinner and less numerous than those of the underlying late Asbian, rarely terminated in emergence, but most reached shallow subtidal depths prior to the next transgression. The change in cyclicity style across the Asbian/Brigantian boundary may be related to the sedimentation rates of the crinoidal limestones, due to increases in cyclic oscillation in the Brigantian. Brigantian LA 4 is characterized by deep subtidal cherty limestones, with abundant algal-coated wackestone intraclasts. Fasciella and Howchinia bradyana are typical microfossils. No cyclicity is observed, probably owing to the deep subtidal nature of this unit. LA 4 is overlain by another unit of cyclic crinoidal limestones (LA 5) in the Burren, which has no correlatives in the other areas studied. The succession in all areas is unconformably overlain by Namurian siliciclastic rocks. The nature and number of minor cycles in the late Dinantian of Ireland is similar to those of platform successions of the same age elsewhere in the British Isles, suggesting that eustatic changes were one of the major controls on cyclicity during the late Dinantian.

Middle to Upper Eocene stratigraphic nomenclature and deposition in the Eucla Basin

The Eucla Basin has the largest onshore extent of Cenozoic marine sediments anywhere in the world. The sediments provide a record of the evolving marine environments of the Southern Ocean and the terrestrial hinterland of the Australian continent. However, owing to its size and remoteness, the Eucla Basin is comparatively understudied. This is exacerbated by the scattered and often deeply weathered nature of the outcrops along the margins of the basin, and the inaccessibility of exposures in the basin centre, except in cliffs and caves. The extent and isolation of the Eucla Basin over two states has resulted in conflicting and overlapping stratigraphic nomenclature, especially of the marginal sediments. Therefore, we propose rationalising the nomenclature of the Eocene rocks in the region based on three guiding principles: the use of consistent terminology across the region; the recognition of the importance of allostratigraphy in defining stratigraphic architecture, in particular two 3rd‐order cycles correlated with the Tortachilla and Tuketja transgressions; and continuity with past usage wherever possible, with a minimum of new terminology. We propose eight major changes to the existing nomenclature: (i) abandoning the term Bremer Basin for the marine and marginal marine to non‐marine Eocene sediments that infill palaeovalleys and form a veneer across crystalline basement in southwest Western Australia and including these sediments in the margin of the Eucla Basin; a similar situation exists in the east, where the Eocene sediments that have been included in the Polda Basin are likewise a marginal extension of the Eucla Basin; (ii) introducing the term Maralinga Formation for all Middle Eocene non‐marine to marginal marine sediments, including those previously included in the lower part of the Pidinga Formation in South Australia, and North Royal Formation for similar sediments in Western Australia; these replace the previous informal usage of lower Pidinga and lower Werillup Formation, respectively; (iii) restricting Hampton Sandstone to its original usage for a calcareous marine sand underlying the Wilson Bluff Limestone; (iv) raising the Paling Member of the Wilson Bluff Limestone to formation status; (v) using Pidinga Formation for all Upper Eocene carbonaceous sediments on the margins of the Eucla Basin in South Australia, and Werillup Formation for all such sediments in Western Australia, including the marginal palaeovalleys; terms such as Wollubar Sandstone in the palaeovalleys of the Yilgarn Craton, and Poelpena and Wanilla Formations in the Eocene part of the former Polda Basin should be abandoned; (vi) using the term Pallinup Formation for all Upper Eocene spicule‐rich sediments along the western margin of the Eucla Basin; (vii) recognising the formation status of the Upper Eocene spicular marine sediments in the eastern Eucla Basin that were formerly termed the Khasta Member of the Hampton Sandstone and the Bring Member of the Pidinga Formation, abandoning the term Bring Member, and including those rocks, and similar sediments of the Poelpena Formation in the Polda Basin, in the new Khasta Formation; and (viii) abandoning the term Toolinna Limestone previously applied to Upper Eocene grainstone along the western margin of the Eucla Basin as it is a facies of the Wilson Bluff Limestone, whereas the grainstone at the type locality at Toolinna Cove is in fact Abrakurrie Limestone and is indistinguishable from the rest of that formation. We believe this rationalisation emphasises the unity of stratigraphy across much of southern Australia and, thus, will facilitate research on the Eucla Basin as a whole.